Funny, Firefox Nightly 55, 56 shows the same font as latest Firefox Nightly 57, but Firefox 56 beta or 55 stable shows the same font as Chrome or Edge. So only difference is in Nightly, no matter number version. It has to be some at least a few months old setting which is valid only for Nightly.No custom font settings.

Edds wrote:Funny, Firefox Nightly 55, 56 shows the same font as latest Firefox Nightly 57, but Firefox 56 beta or 55 stable shows the same font as Chrome or Edge. So only difference is in Nightly, no matter number version. It has to be some at least a few months old setting which is valid only for Nightly.No custom font settings.

If you navigate to Hamburger menu - Library - History - Scroll down to recent history what is the intended behaviour there ? Because nothing is happening when I click on a page from history.edit:The same for Library - Bookmarks